Erica Synths EDU DIY Mixer Eurorack Module Kit - Manual (Page 35)

Before we start building, let’s take a look at the complete mki x es.edu Mixer schematics
(see next page) that were used for the final module’s design and PCB fabrication. Most
components on the production schematics have denominations (a name – like R1, C1, VT1,
VD1, etc.) and values next to them. Denominations help identify each component on the
PCB, which is particularly useful during calibration, modification or troubleshooting.
XS1-XS3 are the Audio input jack sockets, XS4-XS6 are the Audio output jack sockets –
these are the very same we’ve already been using on the breadboard for interfacing with
other devices. In our designs, we use eurorack standard 3,5mm jack sockets (part number
WQP-PJ301M-12).
XP1 is a standard eurorack power connector. It’s a 2x5 male pin header with a key (the
black plastic shroud around the pins) to prevent accidental reverse polarity power supply
connection. This is necessary because connecting the power incorrectly will permanently
damage the module.
VD1 and VD2 are schottky diodes that double-secure the reverse polarity power supply
protection. Diodes pass current only in one direction. Because the anode of VD1 is
connected to +12 V on our power header, it’ll only conduct if the connector is plugged in
correctly. If a negative voltage is accidentally applied to the anode of VD2, it closes, and no
current passes through. The same goes for VD2, which is connected to -12 V. Because
schottky diodes have a low forward voltage drop, they are the most efficient choice for
applications like this.
Next, we have two 10 Ohm resistors (R7 and R8) on the + and – 12 V rails, with decoupling
(or bypass-) capacitors C1 – C4. These capacitors serve as energy reservoirs that keep the
module’s internal supply voltages stable in case there are any fluctuations in the power
supply of the entire modular system. In combination with R7 and R8, the large 47 microfarad
pair (C1 and C2) compensates for low frequency fluctuations, while C3 and C4 filter out
radio frequencies, high frequency spikes from switching power supplies and quick spikes
created by other modules. Often another component – a ferrite bead – is used instead of
a 10 Ohm resistor and there’s no clear consensus among electronic designers which works
best, but generally for analogue modules that work mostly in the audio frequency range (as
opposed to digital ones that use microcontrollers running at 8 MHz frequencies and above),
resistors are considered to be superior.
Another advantage of 10 Ohm resistors is that they will act like slow “fuses” in case there’s
an accidental short circuit somewhere on the PCB, or an integrated circuit (IC) is inserted
backwards into a DIP socket. The resistor will get hot, begin smoking and finally break the
connection. Even though they aren't really fuses, just having them there as fuse substitutes
is pretty useful - you’d rather lose a cent on a destroyed resistor than a few euros on
destroyed ICs.
Capacitors C5 – C8 are additional decoupling capacitors. If you inspect the PCB, you’ll see
that these are placed as close to the power supply pins of the ICs as possible. For well-
designed, larger PCBs you will find decoupling capacitors next to each IC. Like the others,
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MODULE ASSEMBLY APPENDIX